The present invention relates to a monitoring system for electric motors and more particularly to a monitoring system which provides an indication of a motor's useful operating capacity. The monitoring system also allows an historical record to be made of the motor's operating conditions during its working life. The monitoring system can be used with all electric motors, but is particularly intended for use with low cost DC motors of the type commonly found in tail gate lifts, fork lift trucks and the like.
It is known to protect electric motors against overheating by providing simple thermal trips to interrupt the supply of power to the motor whenever the motor temperature exceeds a maximum predetermined level. Typically, the thermal trip takes the form of either a bi-metallic switch which interrupts the supply of power to the motor on thermal overload or a resistive temperature sensor which supplies a current proportional to temperature to a motor cut-out circuit. Both types of thermal trip have short duration memories and operate on fixed temperature levels so that immediately the motor temperature drops back below the maximum predetermined level the thermal overload condition ceases to effect the thermal trip. Moreover, immediately prior to a thermal overload condition arising no warning is given to the operative that the motor is overheating.
Overheating will always have a detrimental effect on the life expectancy of an electric motor and is therefore to be avoided if at all possible. However, the effect of overheating on low cost DC motors (which are typically intended for short duration operations and not continuous use) can be particularly severe; these low cost DC motors are easily damaged if overheated for even a relatively short period of time. Notwithstanding this though it is often the case that low cost DC motors find use in applications where they are sometimes operated close to the maximum "safe" operating capacity. In these circumstances potentially damaging overheating of the motor can occur even though the thermal trip is not actually operated and the operative may not be aware of the abuse to which the motor is being subjected. If the abuse continues to a point where the thermal trip actually cuts out the motor is, of course, disabled, until its temperature has dropped and this can be both inconvenient and dangerous. Consider the case of a fork lift truck which has been operating close to its maximum safe operating capacity for a prolonged period of time. Its motor will have been getting hotter and hotter without the operative necessarily being aware and if the motor cuts out in the middle of a lifting operation this is both inconvenient and potentially hazardous.
Whilst thermal trips can protect a motor against being operated at unacceptably high temperature levels, they do not provide any indication of the motor's useful operating capacity under specific operating conditions. As such no indication is given that the motor is being operated at the extreme of its working capacity or when it is likely to cut-out. Moreover, since different thermal capacity motors must be used to suit different operating conditions, this means that a motor may end up being used in a totally unsuitable application and no indication of this will be provided until the motor actually fails. Finally, with known thermal trips there is no record of any abuse to which a motor has been subjected and because of this the manufacturer may be required to replace a motor (within the terms of warranties and guarantees provided with) it even though it has not been used correctly.